delalloc fragmenting files?

[Eric Sandeen <sandeen@redhat.com>]

I was doing a little seekwatchering today, and found something...
interesting.

I was doing an 8G buffered write via dd, on a machine that reports 3G of
memory, in 1M chunks like so:

dd if=/dev/zero of=/mnt/test/foobar bs=1024k count=8192

on a fairly decent hardware raid, ~90G filesystem.

Kernel is 2.6.24-rc1, with all the git patches from a day or so ago applied.

I made the ext4 fs with lustre's e2fsprogs, with -I 256, and mounted with:

mount -t ext4dev -o data=writeback,delalloc,extents,mballoc /dev/sdb7
/mnt/test

The resulting file had over 4k extents.
[root@bear-05 ~]# filefrag -v /mnt/test/foobar  | grep -i extents
File is stored in extents format
/mnt/test/foobar: 4075 extents found

http://people.redhat.com/esandeen/seekwatcher/ext4-dd-write.png

if I don't mount with delalloc:

mount -t ext4dev -o data=writeback,extents,mballoc /dev/sdb7 /mnt/test

and run the same dd, I get 229 extents:

[root@bear-05 ~]# filefrag -v /mnt/test/foobar  | grep -i extents
File is stored in extents format
/mnt/test/foobar: 229 extents found

http://people.redhat.com/esandeen/seekwatcher/ext4-dd-write-nodelalloc.png

It looks like delalloc is dribbling all over the disk....

(note: times & rates look wrong to me, something is up with blktrace I
think, but FIBMAP shouldn't lie about allocation)

-Eric